Polymorphonuclear leukocytes (PMN) play a crucial role in host protection, however, if not rapidly removed from sites of inflammation cause considerable tissue damage. We postulated that transmigrating PMN leave a "transcriptional imprint" on the cells they migrate across. Our initial findings indicated that infiltrating neutropils (PMN) deplete oxygen to an extent sufficient to generate a hypoxic microenvironment during colitis. Initiation of this microenvironment was dependent on the PMN respiratory burst and resulted in HIF stabilization in surrounding epithelia. Guided by microarray analysis, we identified a number of cytokines regulated by PMN-epithelial crosstalk. These cytokines were regulated in a coordinated, time-dependent fashion and are known to influence the polarity of macrophages. Considering that macrophages are highly plastic cells, influenced by their microenvironment, we hypothesize that the polarity of infiltrating monocytes is influenced by factors secreted in response to PMN-epithelial crosstalk.
Three specific aims are directed at testing this hypothesis:
In Specific Aim 1, we will define the contribution of epithelial hypoxia inducible factor (HIF) to regulating macrophage polarity in vitro and during inflammation in vivo. Specific Aim 2 is designed to elucidate the mechanism behind delayed production of STAT3-dependent anti-inflammatory cytokines in hypoxia.
In Specific Aim 3, we aim to ascertain the functional consequences of crosstalk between macrophages and epithelial HIF/STAT3-dependent factors on restitution of the epithelium.

Public Health Relevance

This proposal aims to gain insight into how an acute inflammatory event can establish a microenvironment to either coordinate the resolution of inflammation, or perpetuate it to generate a chronic inflammatory state. Macrophages are highly plastic cells influenced by their local environment that represent the fulcrum of inflammation, able to shift the balance of inflammation / resolution depending on their activation state. Results from the studies proposed herein will attempt to ascertain the mechanistic influence of epithelial-derived factors that regulate the microenvironment in which macrophages function and ultimately regulate inflammation / resolution.